1. Field of the Invention
The present invention relates to a multilayer piezoelectric composite and, more particularly, to a multilayer piezoelectric composite which is suitable for uses as a whole in which the temperature dependence of the piezoelectric characteristics required to be low, especially, as an electronic circuit element such as a resonance filter and a wave filter.
2. Description of the Prior Art
Piezoelectric materials which enable the mutual conversion between electric energy (electric field) and mechanical energy (strain) are not only utilized for a capacitor and the like as a ferroelectric material but also used as a piezoelectric vibrator for an ultrasonic washer, tuning fork filter, ultrasonic wave oscillator, piezoelectric coupler and ceramic filter, piezoelectric igniting element, piezoelectric element for a transformer, a delaying element, etc., in other words, they are used as a kind of functional materials.
Such piezoelectric materials are classified into three types by their forms, namely, a piezoelectric single crystal, a piezoelectric thin film and a piezoelectric ceramic.
Among these, it is a lead zirconate titanate PbZrO.sub.3 -PbTiO.sub.3 (hereinunder referred to as "PZT") piezoelectric ceramic, which is a two-component solid solution of a ferroelectric material, zirconate and an antiferroelectric material, titanate that is most widely used.
It is known that in the case of a PZT piezoelectric ceramic, when the molar ratio of zirconium and titanium (hereinunder referred to as "Z/T") is 53/47, a morphotropic phase transition from a rhombohedral phase to a tetragonal phase is caused and that piezoelectric constant and an electromechanical coupling factor remarkably increase in the vicinity of the morphotropic phase boundary.
However, in PZT piezoelectric ceramic having a composition approximate to the morphotropic phase boundary, the temperature dependence of the piezoelectric characteristic is high.
FIG. 7 is a graph showing the temperature dependence of frequency constants Np (unit: Hz.multidot.m) of PZT. The numeral in a parenthesis represents a temperature coefficient (TCNp) of a frequency constant Np in a constant temperature range. The temperature coefficient in this specification means the value (unit: ppm) obtained by dividing the difference between the maximum value Np.sub.MAX and the minimum value Mp.sub.MIN of the frequency constant Np in the temperature range of 20.degree. to 100.degree. C. by the temperature difference .DELTA.T and and further dividing the quotient by the frequency constant Np.sub.30 at a temperature 30.degree. C. unless specified otherwise. With respect to PZT, however, since it is curved in a concave, the quotient is divided by Np.sub.20 and N.sub.70 and the temperature coefficients both in the positive direction and in the negative direction are shown.
Additionally, the reason why the temperature range is restricted to below 100.degree. C. is that this range is considered to be a generally practical temperature range for a PZT piezoelectric ceramic. The PZT 52 to the PZT 55 In FIG. 7 represent a single-layer PZT piezoelectric ceramics and the each numeral is equal to the value x in the general formula PbZr.sub.x Ti.sub.1-x representing a PZT piezoelectric ceramic which are multiplied by 100. For example, 100.sub.x =53 represents a PZT piezoelectric ceramic represented by the formula PbZr.sub.0.53 Ti.sub.0.47 O.sub.3 (hereinunder referred to as "PZT 53").
As shown in the parentheses in the graph, the temperature coefficients of the conventional single-layer PZT piezoelectric ceramics are in the order of three figures (PZT 55: -160.times.10.sup.-6, PZT 54: -440.times.10.sup.-6, PZT 53: -345.times.10.sup.-6, +345.times.10.sup.-6, PZT 52: -460.times.10.sup.-6).
Such a piezoelectric ceramic the piezoelectric characteristic of which greatly depends on the temperature is not generally suitable for the use as an electronic circuit element such as a resonance filter and a wave filter.
As a result of studies undertaken by the present inventors so as to solve this problem, it has been found that it is easy to obtain a piezoelectric ceramic having a low temperature dependence without the need for introducing a third component by laminating not less than two piezoelectric ceramics at least one of which has a temperature coefficient having a polarity opposite to the polarity of those of the other layers and which can be sintered at a temperature at which the ceramics form a solid solution, and sintering these ceramics at the comparatively low temperature which do not allow these ceramics to form a solid solution.
The present invention has been achieved on the basis of this finding.